Myasthenia Gravis

History. You examine a 5-year-old female German shepherd whose owner states that the dog becomes progressively weaker with exercise. The owner also states that recently, just after eating, the dog has begun to vomit food in formed, cylinder­shaped boluses.

Clinical Examination. All abnormalities found on physical examination were referable to the neuromuscular system. After resting, the dogs neurological examination find­ings were within normal limits. With even moderate exercise, however, the dog became progressively weaker, particularly in the front legs. Intravenous injection of an acetylcholinesterase inhibitor, edrophonium (Tensilon), eliminated all clinical signs of weakness. Radiographs of the chest revealed an enlarged esophagus and thymus.

Comment. The history of an enlarged esophagus (mega­esophagus) and the response to an acetylcholinesterase inhibitor confirm the diagnosis of myasthenia gravis (“grave muscle weakness”). This is caused by a failure of transmission of acetylcholine at the neuromuscular synapse. This transmis­sion failure is caused by antibodies produced by the body against its own acetylcholine receptors. 'Γhe abnormal anti­bodies bind with the receptors to form complexes, which prevents acetylcholine from binding to the acetylcholine receptors. As a result, no depolarization occurs on the post- synaptic membrane of the cells. Antibodies also alter the junc­tional folds and number of acetylcholine receptors available to bind with the transmitter. Acetylcholinesterase inhibitors pre­vent the metabolism of acetylcholine, allowing acetylcholine to remain longer at the synapse, with additional time for binding to the receptors, and thus facilitating normal transmission.

The large amount of skeletal muscle in the dog’s esophagus explains its enlargement from paralysis. These patients often regurgitate formed boluses of food shortly after eating.

Myasthenia gravis can be associated with mediastinal masses, usually of the thymus. The autoantibodies that the body makes are often against antigens from the thymus or acetylcholine receptors. In addition to this cause of myasthenia gravis, idiopathic myasthenia gravis is also common.

Treatment. Spontaneous remissions are common, depend­ing on the cause. Until then, oral daily acetylcholinesterase inhibitors are given. Surgical removal of mediastinal masses may also be necessary.